1. Field of the Invention
The present invention relates to a reproducing apparatus and a reproducing method that are configured to prevent audio signal clipping.
2. Description of the Related Art
As shown in FIG. 10, an existing reproducing apparatus 100 is mainly composed of a digital signal processing block 111, an analog signal processing block 112, a D/A (Digital/Analog) conversion block 122, and a loudspeaker 134. The existing reproducing apparatus 100 is configured to reproduce digital audio signals received from a storage device 131 such as HDD (Hard Disc Drive), a disc recording media 132 such as CD (Compact Disc) or DVD (Digital Versatile Disc), and an audio signal source such as a tuner 133 that receives television broadcast or radio broadcast.
A digital audio signal outputted from various audio signal sources such as the storage device 131, the disc recording media 132, or the tuner 133 is supplied to the digital signal processing block 111. The digital signal processing block 111, made up of a DSP (Digital Signal Processor) 121, executes various kinds of signal processing on digital audio signals supplied from various audio signal sources, supplying the processed signals to the D/A conversion block 122. The D/A conversion block 122 converts the received digital audio signals into analog audio signals and supplies the analog audio signals to the analog signal processing block 112.
The analog signal processing block 112 is made up of an analog circuit 123, analog volume 124, and an amplifier 125, for example. The analog circuit 123 receives an analog audio signal from the D/A conversion block 122 and executes various kinds of signal processing such as amplification on the received analog audio signal, supplying the processed signal to the analog volume 124. The analog volume 124 amplifies the received analog audio signal on the basis of an operation done by a user through an operating block not shown and supplies the amplified signal to the loudspeaker 134. The loudspeaker 134 outputs the received analog audio signal.
With respect to the above-mentioned reproducing apparatus, Japanese Patent Laid-open No. 2000-174574 discloses a volume cutback apparatus configured to arrange a digital volume and an analog volume, in which the digital volume is adjusted if a volume setting is in excess of a preset threshold and the analog volume is adjusted if a volume setting is below a preset threshold.
Some of the reproducing apparatuses 100 have an effect capabilities for highlighting a particular band among audio signal frequency bands. For these effect capabilities, boost processing is known in which the low frequency side of the frequency band of audio signal is amplified to obtain an effect of highlighting the low frequency side, for example.
Normally, the boost processing for amplifying a signal component of a particular frequency band, such as the low frequency side, may be executed in a method in which a particular frequency band of audio signal is amplified in the analog domain or a method in which a particular frequency band of audio signals amplified in the digital domain.
First, a method of executing boost processing on an audio signal in the analog domain will be described. FIG. 11 shows an exemplary configuration of a reproducing apparatus 100′, in which boost processing is executed in the analog domain. With reference to FIG. 11, components similar to those previously described with reference to the reproducing apparatus 100 shown in FIG. 10 are denoted by the same reference numerals and the description thereof will be skipped.
In this example, an analog circuit 123 arranged in an analog signal processing block 112 is configured to execute boost processing in which the low frequency component of each analog audio signal is amplified. The analog circuit 123 amplifies the low frequency component of an analog audio signal to a predetermined level on the basis of a boost signal supplied from a control block 113 to be described later. An analog volume 124 amplifies the analog audio signal to a predetermined level on the basis of a volume signal supplied from the control block 113.
When a user operates an operating block 114 to turn on boost processing, the control block 113 generates a boost signal for amplifying the low frequency component of an analog audio signal and supplies the generated boost signal to the analog circuit 123. Also, if the user operates the operating block 114 to set the volume to a predetermined level, the control block 113 generates a volume signal for amplifying an analog audio signal in accordance with the set volume and supplies the generated volume signal to the analog volume 124.
FIG. 12 shows the frequency characteristics of audio signals outputted from the DSP 121 that is the digital domain and the analog volume 124 that is the analog domain when boost processing is executed in the analog domain. The graph on the left shows the frequency characteristic of a digital audio signal outputted from the DSP 121. The graph on the right shows the frequency characteristic of an analog audio signal outputted from the analog volume 124.
A digital clip level and an analog clip level over which clipping will occur are preset for each of the audio signals to be entered in the DSP 121 and the analog volume 124.
When the user operates the operating block 114 to raise volume, the analog audio signal is amplified to a predetermined level in the analog volume 124 as shown in FIG. 12, thereby raising the reference level of the analog audio signal relative to the reference level of the digital audio signal. Also, the low frequency component of the analog signal is further amplified to a predetermined level by the boost processing in the analog circuit 123.
The following describes a method of executing boost processing on an audio signal in the digital domain. FIG. 13 shows an exemplary configuration of a reproducing apparatus 100″ for executing boost processing in the analog domain. With reference to FIG. 13, components similar to those previously described with reference to the reproducing apparatus 100 and the reproducing apparatus 100′ shown in FIG. 10 and FIG. 11 are denoted by the same reference numerals and the description thereof will be skipped.
In this example, boost processing for amplifying the low frequency component of each digital audio signal is executed in a DSP 121 arranged in a digital signal processing block 111. The DSP 121 amplifies the low frequency component of a digital audio signal to a predetermined level on the basis of a boost signal supplied from a control block 113.
FIG. 14 shows the frequency characteristics of the audio signals that are outputted from the DSP 121 and an analog volume 124 when boost processing is executed in the digital domain. When the user operates an operating block 114 to turn on boost processing, boost processing is executed on a digital audio signal in the DSP 121 to amplify the low frequency component of the digital audio signal.
At this moment, if the amplitude level of the digital audio signal outputted from the DSP 121 is relatively high and therefore there is little margin up to a digital clip level, the low frequency component amplified by the boost processing may possibly exceed the digital clip level. Therefore, in such a situation, attenuation processing is executed in order to lower the reference level of the digital audio signal once, thereby preventing the low frequency component of the digital audio signal from exceeding the digital clip level, as shown in FIG. 14.
As described above, the low frequency component can be prevented from exceeding the digital clip level in the digital domain by executing boost processing with the reference level of a digital audio signal lowered to amplify the low frequency component of the digital audio signal to a predetermined level.
It should be noted that, in executing boost processing in the digital domain, the reference level of a digital audio signal is lowered by executing attenuation processing and the reference level of an analog audio signal is raised in the analog domain by the amount lowered by the attenuation processing. Hence, there is no variation in the total reference level of the digital domain and the analog domain.
Next, when the user raises volume through the operating block 114, the analog audio signal is amplified to a predetermined level in the analog volume 124, thereby raising the reference level of the analog audio signal relative to the reference level of the digital audio signal.
Thus, in the existing reproducing apparatuses, boost processing is executed on audio signals in the analog domain or the digital domain.